(1) Field of the Invention
The present invention relates to a process for the preparation of a dental crown restoration and a kit for use in carrying out this process. More particularly, the present invention relates to a process for the preparation of a dental crown restoration which can reproduce in an artificial tooth a deep color having good esthetics and resembling the color of a natural tooth and can be easily processed. Furthermore, the present invention relates to an artificial tooth of a novel composition.
(2) Description of the Prior Art
Ceramics now developed as biomaterials, such as alumina, zirconia, carbon, silicon nitride and calcium phosphate ceramics are excellent over conventional anti-corrosive alloys such as stainless steels and nickel-cobalt alloys. Castable ceramics for dental crown restoration materials, that is, crystalline glass ceramics to be applied to cast-forming, are now regarded as being promising. Since the material of this type is prepared by the lost-wax process, reproduction can be accomplished more easily than in the conventional porcelain build-up process and even a fine part can be reproduced faithfully, and a high experience technique becomes unnecessary.
The specification of U.S. Pat. No. 4,650,418 discloses a dental prosthetic facing comprising an outer shader layer, an intermediate at least substantially uncolored translucent layer, and an opaque substrate color-keyed with said shader to match a shade guide. It is taught that this uncolored translucent layer is formed of mica ceramic glass.
Furthermore, the specification of U.S. Pat. No. 4,189,325 discloses a glass-ceramic dental restoration comprising LiO.sub.2, CaO, Al.sub.2 O.sub.3 and it is taught that the restoration can be colored by incorporation of one or more inorganic coloring agents.
Moreover, European Patent Application 0022655 discloses a prosthetic material formed by casting and crystallizing a glass ceramic of the mica composition, and it is taught that various colorants can be incorporated into this ceramic.
Still further, Japanese Patent Application Laid-Open Specification No. 69007/85 discloses an artificial dental crown composed of a glass ceramic in which crystallites of a calcium phosphate type mineral composed mainly of apatite are formed, and it is taught that the back surface of the dental crown is colored with a coloring agent. Japanese Patent Application Laid-Open Specification No. 96544/85 teaches that a crystallized glass of the calcium phosphate type composed mainly of apatite and having a Ca/P atomic ratio of 0.35 to 1.7 is colored by incorporating one or more transition metal oxides and rare earth element oxides.
Generally, 16 to 24 kinds of shade guides are used for making the shade of an artificial tooth inclusive of a dental crown in agreement with or closely akin to the shade of an adjacent tooth.
According to the above-mentioned teaching of U.S. Pat. No. 4,650,418, the outer shader layer is combined with the opaque substrate through the translucent intermediate layer to produce a color matched with a shade guide. However, even according to this teaching, it is still difficult to reproduce a deep color resembling the color of a natural tooth. More specifically, the interior of a natural tooth is composed of colored dentine and the surface is composed of substantially uncolored transparent enamel. In contrast, the above-mentioned artificial tooth has a reverse structure and a color lacking depth is likely formed. Moreover, in order to produce a predetermined color in the outer shader layer, the operation of coating a shading composition and firing the coating should be repeated many times, for example, at least 6 times, and the processing operation becomes complicated. Moreover, if the shader layer is worn, the uncolored translucent layer is exposed and the color of the artificial tooth is drastically changed, and the allowance of the durability or life of the artificial tooth is relatively narrow.
According to the conventional technique of coloring a glass ceramic per se, an artificial tooth having a good durability and a color akin to the color of a natural tooth can be stably prepared, but a variety of pigment-incorporated glass ceramic compositions should be prepared according to the shade guides.
In the conventional crystallized glass ceramic of the calcium phosphate type composed mainly of apatite and having a Ca/P atomic ratio of 0.35 to 1.7, opacification or whitening by the crystallization is conspicuous, and the appearance of the ceramic is quite different from the appearance of a natural tooth. Moreover, even if this apatite type ceramic is colored by internal addition of a pigment or by coating with a shading composition, it is difficult to reproduce a somewhat translucent appearance of a natural tooth.